Electronic device, method and computer program

ABSTRACT

An electronic device comprising circuitry configured to drive a unit pixel for a time of flight camera according to a multi-level mixing clock scheme.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims priority to European Patent Application18182863.3 filed by the European Patent Office on Jul. 11, 2018, theentire contents of which being incorporated herein by reference.

TECHNICAL FIELD

The present disclosure generally pertains to the field of electronicdevices, in particular imaging devices and methods for imaging devices.

TECHNICAL BACKGROUND

A time-of-flight camera is a range imaging camera system that determinesthe distance of objects measuring the time-of-flight (ToF) of a lightsignal between the camera and the object for each point of the image. Atime-of-flight camera thus receives a depth map of a scene. Generally, atime-of-flight camera has an illumination unit that illuminates a regionof interest with modulated light, and a pixel array that collects lightreflected from the same region of interest. As individual pixels collectlight from certain parts of the scene, a time-of-flight camera mayinclude a lens for imaging while maintaining a reasonable lightcollection area.

A typical ToF camera pixel develops a charge that represents acorrelation between the illuminated light and the backscattered light.To enable the correlation between the illuminated light and thebackscattered light, each pixel is controlled by a common modulationinput coming from one or more mixing drivers. The modulation input tothe pixels is synchronous with an illumination block modulation.

The load of the mixing drivers is typically capacitive. The powerconsumed is described by the well-known equation CV²f, where C is thetotal load capacitance, V is the supply voltage and f is the switchingspeed of the mixing drivers (or modulation frequency). The mixingdrivers consume a lot of power especially when the load capacitance islarge or the modulation frequency is high.

Conventionally the power consumption is reduced by reducing the loadcapacitance, especially by reducing the photo-gate/transfer-gatecapacitance.

SUMMARY

According to a first aspect, the disclosure provides an electronicdevice comprising circuitry configured to drive a unit pixel for a timeof flight camera according to a multi-level mixing clock scheme.

According to a second aspect, the disclosure provides a method,comprising driving a unit pixel for a time of flight camera according toa multi-level mixing clock scheme.

According to a third aspect, the disclosure provides a time-of-flightsystem comprising the circuitry of the first aspect, a light source andan image sensor.

According to a fourth aspect, the disclosure provides a computerprogram, comprising instructions, the instructions when executed on aprocessor controlling a driver of a unit pixel for a time of flightcamera according to a multi-level mixing clock scheme.

Further aspects are set forth in the dependent claims, the followingdescription and the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments are explained by way of example with respect to theaccompanying drawings, in which:

FIG. 1 illustrates schematically the basic operational principle of anindirect time-of-flight (iToF);

FIG. 2 shows a circuitry of a conventional mixing driver of a ToF camerawith a one column pixel array;

FIG. 3 shows modulation signals that are supplied to the inputs of themixing driver of FIG. 2;

FIG. 4 shows a first embodiment of a circuitry of a mixing driver of aToF camera with an active two-level mixing clock scheme;

FIG. 5 shows a multi-level clock scheme for driving six switches of themixing driver of FIG. 4, as well as effective modulation signalwaveforms in the time domain;

FIG. 6 shows a second embodiment of a circuitry of a mixing driver for aToF camera with an active N-level mixing clock scheme;

FIG. 7 shows a multi-level clock scheme for driving the switches of themixing driver of FIG. 6;

FIG. 8 shows a third embodiment of a circuitry of a mixing driver of aToF camera with a passive two-level mixing clock scheme; and

FIG. 9 shows a multi-level clock scheme for controlling a switch anddigital buffers of the mixing driver of FIG. 8, as well as the effectivemodulation signal clock waveforms in time domain.

DETAILED DESCRIPTION OF EMBODIMENTS

Before a detailed description of a first embodiment of the presentdisclosure under reference of FIG. 3 is given, general explanations aremade.

As mentioned in the outset, time-of-flight (ToF) cameras are known toinclude a variety of methods that measure the time that light needs fortravelling a distance in a medium, such that the distance can bedetermined. In indirect time-of-flight (iToF) cameras calculate a phaseshift between illuminated light and backscattered light for obtainingdepth measurements by sampling a correlation wave, e.g. between amodulation signal for driving a light source, pixel arrays, or the like,with a signal obtained based on backscattered light.

The embodiments described below provide an electronic device comprisingcircuitry configured to drive a unit pixel for a time of flight cameraaccording to a multi-level mixing clock scheme.

The electronic device may for example be an image sensor, e.g. an imagesensor of an in direct time of flight camera (ToF). An indirect time offlight camera may resolve distance by measuring a phase shift of anemitted light and a back scattered light.

Circuitry may include any electronic elements, semiconductor elements,switches, amplifiers, transistors, processing elements, and the like.

The circuitry may in particular be a driver for ToF unit pixels whichprovides a modulated signal to the signal inputs of one or more unitpixels. Driving a unit pixel of a time of flight camera with multi-levelmixing clock signals may for example comprise using the multi-levelmixing clock signals as modulation signals for the unit pixel. Amodulation signal may be a signal which is correlated to the signalcollected in the unit pixel.

A time-of-flight camera may be a range imaging camera system thatdetermines the distance of objects measuring the time-of-flight (To F)of a light signal between the camera and the object for each point ofthe image.

The unit pixels of a ToF camera typically comprise one or morephotosensitive elements (e.g. photo diodes). A photosensitive elementconverts the incoming light into a current. Switches (e.g. transfergates) that are connected to the photo diode may direct the current toone or more memory elements (e.g. capacitors) that act as accumulationelements that accumulate and/or store charge. The unit pixels may belock-in pixels, e.g. a FDGS type pixels or Photonic Mixer Devices (PMD),for the time of flight camera. All unit pixels in the ToF sensor may becontrolled by the modulation signal which is based on the multi-levelmixing clock signal.

The multi-level mixing clock scheme may be used to generate one or more(effective) modulation signals that drive the unit pixels. These(effective) modulation signals may be step functions that comprisesmultiple voltage levels.

In some embodiments, the unit pixel comprises a first trace and a secondtrace and wherein the multi-level mixing scheme comprises supplying aneffective first trace modulation signal to the first trace of the unitpixel and supplying an effective second trace modulation signal to thesecond trace of the unit pixel.

For example, the first trace and the second trace may compriserespective storage capacitors of a unit pixel that are charged anddischarged by the effective modulation signal.

Typically, the effective first trace modulation signal and the effectivesecond trace modulation signal are 180° phase shifted.

The effective modulation signals may for example have a frequency in therange of 10 to 100 MHz.

In some embodiments, the multi-level mixing clock scheme is an N-levelmixing clock scheme, wherein the effective modulation signals have N+1voltage levels, where N is an integer number greater than 1.

The effective modulation signals may for example be signals thatoscillate between a high state V_(DD) and a low state GND with N+1voltage levels, where N is an integer number greater than 1, where avoltage step is a voltage transition from a voltage level to anothervoltage level.

The effective modulation signal may be a periodic signal, wherein aperiod comprises a charging phase and a discharging phase, where each ofthe phases includes N voltage steps.

In some embodiments, in the N-level mixing clock scheme the N+1 voltagelevels of the effective modulation signals define N voltage steps.

In some embodiments, the multi-level mixing clock scheme is a two-levelmixing clock scheme, wherein the effective modulation signals providesthree voltage levels.

The effective modulation signals may for example be signals thatoscillate between GND and V_(DD) with an intermediate voltage levelV_(DD)/2. In this embodiment with three voltage-levels (GND, V_(DD)/2,V_(DD)), there are two voltage steps, namely GND to V_(DD)/2, andV_(DD)/2 to V_(DD). The effective modulation signal of threevoltage-levels according to this embodiment may be a periodic signal,wherein a period comprises a charging phase and a discharging phase,where each of the phases include two voltage steps.

In some embodiments, the multi-level mixing scheme is an activemulti-level mixing scheme.

An active multi-level mixing scheme may comprise the providing ofseveral predefined voltage levels and the generating of an effectivemodulation signal from these predefined voltage levels.

In some embodiments, the active multi-level mixing scheme comprises thegenerating of an effective first trace modulation signal and aneffective second trace modulation signal from predefined voltage levels.

In some embodiments, the circuitry comprises switches which are drivenaccording to the multi-level mixing scheme to generate the effectivefirst trace modulation signal and the effective second trace modulationsignal.

In some embodiments, the voltage levels are provided by analog buffersto the unit pixel.

The analog buffers may deliver a voltage of the multi-level mixingscheme to the unit pixels.

In some embodiments, the multi-level mixing scheme is a passivemulti-level mixing scheme.

In some embodiments, the passive multi-level mixing scheme comprisespassively redistributing charge between a first trace and a second traceof the unit pixel to generate the effective first trace modulationsignal and the effective second trace modulation signal.

For example, the passive multi-level mixing scheme may comprisepassively redistributing charge between one or more first storagecapacitors of a first trace and one or more second storage capacitors ofa second trace of the unit pixel to generate the effective first tracemodulation signal and the effective second trace modulation signal.

In some embodiments, the circuitry comprises a switch that is configuredto connect a first trace of the unit pixel with a second trace of theunit pixel for passively redistributing charge between the first traceand the second trace of the unit pixel.

For example, the circuitry may comprise a switch that is configured toconnect one or more first capacitors of a first trace of the unit pixelwith one or more second capacitors of a second trace of the unit pixelfor passively redistributing charge between the first trace and thesecond trace of the unit pixel.

By controlling the switch, the circuitry may control and manage thetiming between the multi-level mixing signal and the voltage/current(effective modulation signal) as seen by the unit pixels.

In some embodiments, the circuitry comprises a first digital buffer fordriving a first trace of the unit pixel and a second digital buffer fordriving a second trace of the unit pixel, and wherein to the firstbuffer a first trace modulation signal is supplied and wherein to thesecond buffer a second trace modulation signal is supplied.

For example, the first trace modulation signal and the second tracemodulation signal are phase shifted by 180°. Although more complexarrangements may be used, the modulation signals may be a square wavewith a frequency range of 10 to 100 MHz.

In some embodiments, the first digital buffer and the second digitalbuffer are enabled/disabled according to the multi-level mixing schemeto generate the effective first trace modulation signal and theeffective second trace modulation signal.

The digital buffers may be tri-state buffers, i.e. an input controlledswitch which comprises an input, an output and a control input. Theoutput may be electronically turned “ON” or “OFF” by means of anexternal enable/disable control input. This control signal input may beeither a logic “0” or a logic “1”.

The embodiments also disclose a time-of-flight system comprising thecircuitry according to the embodiments, a light source and an imagesensor.

The embodiments also disclose a method, comprising driving a unit pixelfor a time of flight camera according to a multi-level mixing clockscheme.

The embodiments also disclose a computer program, comprisinginstructions, the instructions when executed on a processor controllinga driver of a unit pixel for a time of flight camera according to amulti-level mixing clock scheme.

FIG. 1 illustrates schematically the basic operational principle of anindirect time-of-flight (iToF) camera. The iToF camera includes anillumination unit (laser) 2, a lens 3 and an iToF sensor 6. The iToFsensor 6 includes a time resolved pixel 7 array. The time resolved pixel7 includes two floating diffusions FA, FB, two mixing clock guiders GA,GB and a photo diode 8. The mixing clock guiders GA, GB are controlledby a synchronized clock with a modulation clock in the illumination unit(laser) 2. The photo diode 8 generates electrons based on incidentphotons. The generated electrons are guided to the floating diffusion FAor to the floating diffusion FB, since complementary clocks are appliedto the two mixing clock guiders GA and GB.

An object 1 is actively illuminated with a modulated light 4 at apredetermined wavelength using the dedicated illumination unit 2, forinstance with some light pulses of at least one predetermined frequencygenerated by a timing generator (not shown in FIG. 1). The modulatedlight 4 is returned from the object 1. A lens 3 collects the returninglight 5 and forms an image of the objects onto the iToF sensor 4 of thecamera. Depending on the distance Z of objects from the camera, a delayis experienced between the emission of the modulated light 4, e.g. theso-called light pulses, and the reception at the camera of thosereturned light pulses 5.

Indirect time-of-flight (iToF) cameras calculate a delay betweenmodulated light 4 and returned light 5 for obtaining depth measurementsby sampling a correlation wave, e.g. between a demodulation signalgenerated by the timing generator and the reflected light 5 that isstored in a time resolved pixel 7.

FIG. 2 shows, as an example, a circuitry of a conventional mixing driverof a ToF camera with a one column pixel array. The mixing driver has twoinputs I1, I2, two buffers BA, BB and several unit pixels UP1, . . . ,UPN. Each unit pixel has storage capacitances C1, C2, C3, C4 andintegration capacitance C5, C6. The storage capacitances C1, C2, C3, C4and integration capacitances C5, C6 build together a load capacitancethat is periodically charged and discharged. Resistors R1, R2 are placedbetween storage capacitances C1, C2, and, respectively, C3, C4.Modulation signals GDA, GDB are supplied to the input I1 and the inputI2. The supplied modulation signals GDA, GDB are delivered to the unitpixels UP1, . . . , UPN by buffers BA, BB. The input of buffer BA isconnected to the first input I1 and the output of buffer BA is connectedto the upper trace of the unit pixel P1. The input of buffer BB isconnected to the second input I2 and the output of buffer BB isconnected to the lower trace of the unit pixel P1. The buffers BA, BBare used to isolate the input I1, I2 from the output.

FIG. 3 shows the modulation signals GDA, GDB that are supplied to theinputs I1, I2 of the mixing driver of FIG. 2. The voltages of themodulation signals GDA, GDB change from ground GND to V_(DD)periodically, and the two modulation signals GDA, GDB have a phase shiftof 180 degree. Frequencies used for the modulation signals may be in therange of 10-100 MHz.

A total average power consumption of the mixing driver of FIG. 2 duringthe charging and discharging of the load capacitance may be calculatedusing the equation:

P=C _(total) ×V _(DD) ² ×f,

where C_(total) is the total load capacitance, V_(DD) is the supplyvoltage and f is the switching speed of the mixing drivers (ormodulation frequency).

Active Multi-Level Mixing

FIG. 4 shows, as an example, a first embodiment of a circuitry of amixing driver of a ToF camera with an active two-level mixing clockscheme. The mixing driver has four inputs I1, I2, I3, I4, four analogbuffers BA1, BA2, BB1, BB2, two grounds GND1, GND2, six switches SA1,SA2, SA3, SB1, SB2, SB3 and N unit pixels UP1, . . . , UPN. The unitpixels UP1, . . . , UPN have an identical structure as shown in FIG. 2.

The first input I1 supplies an input voltage of V_(DD), the second inputI2 supplies an input voltage of V_(DD)/2, the third input I3 supplies aninput voltage of V_(DD) and the fourth input I4 supplies an inputvoltage of V_(DD)/2.

The mixing driver has an upper trace and a lower trace. The upper tracecomprises the first input I1, the second input I2 and the first groundGND1 that are connected to several unit pixels UP1, . . . , UPN throughthe analog buffers BA1, BA2. Further, the first input I1, the secondinput I2 and the first ground GND1 are connected with switch SA1, switchSA2 and switch SA3, respectively.

The lower trace has a similar configuration as the upper trace, wherethe lower trace comprises the third input I3, the fourth input I4 andthe second ground GND2 that are connected to several unit pixels UP1, .. . , UPN through the analog buffers BB1, BB2. Further, the third inputI3, the fourth input I4 and the second ground GND2 are connected withswitch SB1, switch SB2 and switch SB3, respectively.

By turning on/off the switches SA1, SA2, SA3, SB1, SB2, SB3 therespective voltages of the respective inputs are applied to the uppertrace, and, respectively, the lower trace of unit pixels UP1, . . . ,UPN.

FIG. 5 shows, as an example, a multi-level clock scheme for driving thesix switches SA1, SA2, SA3, SB1, SB2, SB3, as well as the effectivemodulation signal waveform Effective_GDA of the upper trace on pixel,and the effective modulation signal waveform Effective_GDB of the lowertrace on pixel in time domain resulting from this multi-level clockscheme.

In a first phase T1, switch SA1 and switch SB3 are turned on andswitches SA2, SA3, SB1, SB2 are turned off. The effective modulationsignal Effective_GDA of the upper trace thus is V_(DD) and the effectivemodulation signal Effective_GDB of the lower trace is GND. Therefore, avoltage with an amplitude of V_(DD) is applied to the pixel array by theupper trace and GND is applied to the to the pixel array by the lowertrace. In a second phase T2, switch SA2 and switch SB2 are turned on andswitches SA1, SA3, SB1, SB3 are turned off. The effective modulationsignal Effective_GDA of the upper trace thus is V_(DD)/2 and theeffective modulation signal Effective_GDB of the lower trace isV_(DD)/2. In a third phase T3, switch SA3 and switch SB1 are turned onand switches SA1, SA2, SB2, SB3 are turned off. The effective modulationsignal Effective_GDA of the upper trace thus is GND and the effectivemodulation signal Effective_GDB of the lower trace is V_(DD). In afourth phase T4, switch SA2 and switch SB2 are turned on and switchesSA1, SA3, SB1, SB3 are turned off. The effective modulation signalEffective_GDA of the upper trace thus is V_(DD)/2 and the effectivemodulation signal Effective_GDB of the lower trace is V_(DD)/2.

The phases T1 to T4 are repeated so that the resulting effectivemodulation signal of the upper trace/lower trace results is a two-stepfunction as shown in FIG. 5.

Taking the upper trace as an example, when it is charged, instead ofbeing charged from GND to V_(DD) directly, it is first charged from GNDto V_(DD)/2 and then from V_(DD)/2 to V_(DD).

In the step from the third phase T3 to the fourth phase T4 (chargingphase), i.e. from GND to V_(DD)/2 (effective upper trace modulationsignal), the power consumption is

P1=0.5×V _(DD) ×C _(L)×0.5×V _(DD) ×f=0.25×C _(total) ×V _(DD) ² ×f

In the step from the fourth phase T4 to the first phase T1 (chargingphase), i.e. from V_(DD)/2 to V_(DD), the power consumption is

P2=V _(DD) ×C _(L)×0.5×V _(DD) ×f=0.5×C _(L) ×V _(DD) ² ×f.

Therefore, the total average power consumption is

P _(total) =P1+P2=0.75×C _(L) ×V _(DD) ² ×f.

Compared to the total average power consumption of the conventionalmethod that is shown in FIG. 2 the total average power consumption savedby the first embodiment is:

P _(save)=0.25×C _(total) ×V _(DD) ² ×f.

That 25% power is saved compared to the conventional method.

FIG. 6 shows, as an example, a second embodiment of a circuitry of amixing driver for a ToF camera with an active N-level mixing clockscheme. The mixing driver has 2N inputs AI1, . . . , AIN and BI1, . . ., BIN, 2N analog Buffers BA1, . . . , BAN and BB1, . . . , BBN, twogrounds GND1, GND2, 2(N+1) switches SA1, . . . , SAN+1 and SB1, . . . ,SBN+1 and several unit pixels UP1, . . . , UPN.

The mixing driver 2 has an upper trace and a lower trace. The uppertrace comprises N inputs AI1, . . . , AIN and the first ground GND1 thatare connected to several unit pixels UP1, . . . , UPN through the analogbuffers BA1, . . . , BAN. Further, the N inputs AI1, . . . , AIN and thefirst ground GND1 are connected with the N+1 switches SA1, . . . ,SAN+1. By turning on/off the switches SA1, . . . , SAN+1 of therespective inputs AI1, . . . , AIN the respective voltage of the inputsAI1, . . . , AIN is applied to the unit pixels UP1, . . . , UPN. Thefirst input voltage AI1 of the upper trace has an amplitude of V_(DD),the second input voltage AI2 of the upper trace has an amplitude ofV_(DD)/2, . . . , the N−1 input voltage AIN−1 of the upper trace has anamplitude of V_(DD)×(N−1)/N and the N input voltage AIN of the uppertrace has an amplitude of V_(DD)×1/N.

The lower trace has a similar configuration as the upper trace, wherethe lower trace comprises N inputs BI1, . . . , BIN and the secondground GND2 that are connected to several unit pixels UP1, . . . , UPNthrough the analog buffers BB1, . . . , BBN. Further, the N inputs BI1,. . . , BIN and the second ground GND1 are connected with the N+1switches SB1, . . . , SBN+1. By turning on/off the switches SB1, . . . ,SBN+1 of the respective inputs BI1, . . . , BIN the respective voltageof the inputs BI1, . . . , BIN is applied to the unit pixels UP1, . . ., UPN. The first input voltage BI1 of the lower trace has an amplitudeof V_(DD), the second input voltage BI2 of the lower trace has anamplitude of V_(DD)/2, . . . , the N−1 input voltage BIN−1 of the lowertrace has an amplitude of V_(DD)×(N−1)/N and the N input voltage BIN ofthe lower trace has an amplitude of V_(DD)×1/N.

FIG. 7 shows, as an example, a multi-level clock scheme for driving the2(N+1) switches SA1, . . . , SAN+1 and SB1, . . . , SBN+1 of FIG. 6. Theeffective modulation signal waveform Effective_GDA of the upper trace onthe pixel, and the effective modulation signal waveform Effective_GDB ofthe lower trace on the pixel in time domain are not shown in FIG. 7,however, the effective modulation signal waveform Effective_GDA,Effective_GDB have a similar shape as in FIG. 5.

In a first phase T1, switch SA1 and switch SBN+1 are turned on and theremaining switches are turned off. The effective modulation signal ofthe upper trace thus is V_(DD) and the effective modulation signal ofthe lower trace is GND. In a second phase T2, switch SA2 and switch SBNare turned on and the remaining switches are turned off. The effectivemodulation signal of the upper trace thus is V_(DD)×(N−1)/N and theeffective modulation signal of the lower trace is V_(DD)×1/N. Thisscheme is repeated until in a phase TN, switch SAN and switch SB2 areturned on and the remaining switches are turned off. The effectivemodulation signal of the upper trace thus is V_(DD)×1/N and theeffective modulation signal of the lower trace is V_(DD)×(N−1)/N. In afinal phase TN+1, switch SAN+1 and switch SB1 are turned on and theremaining switches are turned off. The effective modulation signal ofthe upper trace thus is GND and the effective modulation signal of thelower trace is V_(DD).

The phases T1 to TN+1 are repeated so that the resulting effectivemodulation signal of the upper trace/lower trace results is a N-stepfunction (not shown in FIG. 7), where the upper trace and the lowertrace has a 180 degree phase difference.

With a similar calculation as in FIGS. 3 and 4 the total average powerconsumption of the mixing driver with an active N-level mixing clockscheme is

P _(save)=(N−1)/2N×C _(total) ×V _(DD) ² ×f.

That is, compared to the total average power consumption of theconventional method that is shown in FIG. 2, (N−1)/2N % of the power issaved.

Passive Multi-Level Mixing

FIG. 8 shows, as an example, a third embodiment of a circuitry of amixing driver of a ToF camera with a passive two-level mixing clockscheme. The mixing driver has two inputs I1, I2, two digital buffersDBA, DBB, a switch S0 and several unit pixels UP1, . . . , UPN.

The digital buffers DBA, DBB comprise an input, an output and a controlinput. The digital buffers DBA, DBB are turned on or off by the controlinput. When the digital buffers DBA, DBB are turned on (enable) theinput signal is delivered to the output, and when the digital buffersDBA, DBB are turned off (disable) the input signal is not delivered tothe output.

The mixing driver has an upper trace and a lower trace. The upper tracecomprises the first input I1 to which a first modulation signal GDA issupplied. The first input I1 is connected to several unit pixels UP1, .. . , UPN through the first digital buffer DBA.

The lower trace has a similar configuration as the upper trace, wherethe lower trace comprises the second input I2 to which a secondmodulation signal GDB is supplied. The second input I2 is connected toseveral unit pixels UP1, . . . , UPN through the second passive bufferDBB.

The switch S0 is located in front of the several unit pixels UP1, . . ., UPN, and connects the upper trace and the lower trace.

Each unit pixel has storage capacitances C1, C2, C3, C4 and integrationcapacitance C5, C6. The storage capacitances C1, C2, C3, C4 andintegration capacitance C5, C6 builds a load capacitance that isperiodically charged and discharged from ground to supply power V_(DD)that is introduced by the passive buffers DBA, DBB. Resistors R1, R2 areplaced between storage capacitances C1, C2, and, respectively, C3, C4.

FIG. 9 shows a multi-level clock scheme for controlling a switch anddigital buffers of the mixing driver of FIG. 8, as well as the effectivemodulation signal clock waveforms in time domain. In particular, FIG. 9shows, as an example, a multi-level clock scheme with modulation signalsGDA, GDB, a control signal for switch S0, an enable signal of digitalbuffers DBA, DBB, as well as the effective modulation signalEffective_GDA supplied to the upper trace of the pixel, and theeffective modulation signal Effective_GDB supplied to the lower trace ofthe pixel in time domain resulting from this multi-level clock scheme.

The modulation signals GDA, GDB at the input of the mixing driver changefrom ground to V_(DD) periodically, and the two-modulation signals GDA,GDB have a phase shift of 180 degree with respect to each other.

In a first phase T1, the first modulation signal GDA and the secondmodulation signal GDB are at GND. Further, the switch S0 is on and thedigital buffers DBA, DBB are disabled. While the switch S0 is on, thedigital buffers DBA, DBB are disabled to create high impedance at theoutput of the buffers DBA, DBB, and thus the charge on lower trace,which was charged to V_(DD), is passively redistributed to the uppertrace that was discharged to GND. Since the number of unit pixels onboth traces is very large, the total capacity C_(total) of the twotraces is the same, therefore, half of the charged voltages arepassively transferred to the low voltage side without consuming power.Therefore, a voltage with an amplitude of V_(DD)/2 establishes at theupper trace of the pixel array and a voltage with an amplitude of GNDestablishes at the lower trace of the pixel array. In a second phase T2,the first modulation signal GDA is driven high (V_(DD)) and the secondmodulation signal GDB remains at GND. Further, the switch S0 is off andthe digital buffers DBA, DBB are enabled. Thus, a voltage with anamplitude of V_(DD) establishes at the upper trace of the pixel arrayand a voltage with an amplitude of GND establishes at the lower trace ofthe pixel array. In a third phase T3, the first modulation signal GDAand the second modulation signal GDB are at GND. Further, the switch S0is on and the digital buffers DBA, DBB are disabled. The voltage that ischarged by the upper trace is passively redistributed to the lowertrace. Therefore, a voltage with an amplitude of V_(DD)/2 establishes atthe upper trace of the pixel array and a voltage with an amplitude ofV_(DD)/2 establishes at the lower trace of the pixel array. In a fourthphase T4, the first modulation signal GDA remains at GND and the secondmodulation signal GDB is driven high (V_(DD)). Further, the switch S0 isoff and the digital buffers DBA, DBB are enabled. Therefore, a voltagewith an amplitude of GND establishes at the upper trace of the pixelarray and a voltage with an amplitude of V_(DD) establishes at the lowertrace of the pixel array.

The steps T1 to T4 are repeated so that the resulting effectivemodulation signal of the upper trace/lower trace results is a two-stepfunction as shown in FIG. 9.

As shown in FIG. 9, the effective modulation signal Effective_GDA thatestablishes on the of the upper trace of pixel units, and the effectivemodulation signal Effective_GDB that establishes on the of the lowertrace of pixel units have two steps from GND to V_(DD) (GND to V_(DD)/2,V_(DD)/2 to V_(DD)) and the upper trace and the lower trace have a 180degree phase difference with respect to each other.

In the step from the fourth phase T4 to the first phase T1 (firstpassive charging phase), i.e. from GND to V_(DD)/2 (effective uppertrace modulation signal), the power consumption of this step is

P1=0.

In the step from the first phase T1 to the second phase T2 (secondpassive charging phase), i.e. from V_(DD)/2 to V_(DD), the powerconsumption of this step is

P2=V _(DD) ×C _(L)×0.5V _(DD) ×f=0.5×C _(total) ×V _(DD) ² ×f.

Therefore, the total average power consumption is

P _(total) =P1+P2=0.5×C _(total) ×V _(DD) ² ×f.

Compared to the total average power consumption of the conventionalmethod that is shown in FIG. 2 the total average power consumption savedby the third embodiment is:

P _(save)=0.5×C _(total) ×V _(DD) ² ×f.

That 50% power is saved compared to the conventional method.

FIGS. 7 and 8 described above present a passive two-level mixing clockscheme. However, the disclosure is not restricted to a passive two-levelmixing clock scheme, but with the same principles it is also possible toprovide a multi-level mixing clock scheme with more than two levels.

All units and entities described in this specification and claimed inthe appended claims can, if not stated otherwise, be implemented asintegrated circuit logic, for example on a chip, and functionalityprovided by such units and entities can, if not stated otherwise, beimplemented by software.

Insofar as the embodiments of the disclosure described above areimplemented, at least in part, using software-controlled data processingapparatus, it will be appreciated that a computer program providing suchsoftware control and a transmission, storage or other medium by whichsuch a computer program is provided are envisaged as aspects of thepresent disclosure.

Note that the present technology can also be configured as describedbelow.

(1) An electronic device comprising circuitry configured to drive a unitpixel (UP1, . . . , UPN) for a time of flight camera according to amulti-level mixing clock scheme.

(2) The electronic device of (1), wherein the unit pixel (UP1) comprisesa first trace (C1, C2) and a second trace (C3, C4) and wherein themulti-level mixing scheme comprises supplying an effective first tracemodulation signal (Effective_GDA) to the first trace (C1, C2) of theunit pixel (UP1) and supplying an effective second trace modulationsignal (Effective_GDB) to the second trace (C3, C4) of the unit pixel(UP1).

(3) The electronic device of (1), wherein the multi-level mixing clockscheme is an N-level mixing clock scheme, wherein the effectivemodulation signals (Effective_GDA, Effective_GDB) have N+1 voltagelevels, where N is an integer number greater than 1.

(4) The electronic device of (3), wherein in the N-level mixing clockscheme the N+1 voltage levels of the effective modulation signals(Effective_GDA, Effective_GDB) define N voltage steps.

(5) The electronic device of anyone of (2) to (4), wherein themulti-level mixing clock scheme is a two-level mixing clock scheme,wherein the effective modulation signals (Effective_GDA, Effective_GDB)provides three voltage levels (GND, V_(DD)/2, V_(DD)).

(6) The electronic device of anyone of (1) to (5), wherein themulti-level mixing scheme is an active multi-level mixing scheme.

(7) The electronic device of (6), wherein the active multi-level mixingscheme comprises generating an effective first trace modulation signal(Effective_GDA) and an effective second trace modulation signal(Effective_GDB) from predefined voltage levels (V_(DD), V_(DD)×(N−1)/N,. . . , V_(DD)×1/N, GND).

(8) The electronic device of (6) or (7), wherein the circuitry comprisesswitches (SA1, . . . , SAN+1, SB1, . . . , SBN+1) which are drivenaccording to the multi-level mixing scheme to generate the effectivefirst trace modulation signal (Effective_GDA) and the effective secondtrace modulation signal (Effective_GDB).

(9) The electronic device of anyone of (6) to (8), wherein the voltagelevels (V_(DD), V_(DD)×(N−1)/N, . . . , V_(DD)×1/N, GND) are provided byanalog buffers (BA1, BA1, BB1, BB2; BA1, . . . , BAN, BB1, . . . , BBN)to the unit pixel (UP1, . . . UPN).

(10) The electronic device of anyone of (1) to (5), wherein themulti-level mixing scheme is a passive multi-level mixing scheme.

(11) The electronic device of (10), wherein the passive multi-levelmixing scheme comprises passively redistributing charge between a firsttrace (C1, C2) and a second trace (C3, C4) of the unit pixel (UP1, . . ., UPN) to generate the effective first trace modulation signal(Effective_GDA) and the effective second trace modulation signal(Effective_GDB).

(12) The electronic device of (10) or (11), wherein the circuitrycomprises a switch (S0) that is configured to connect a first trace (C1,C2) of the unit pixel (UP1) with a second trace (C3, C4) of the unitpixel for passively redistributing charge between the first trace (C1,C2) and the second trace (C3, C4) of the unit pixel (UP1).

(13) The electronic device of anyone of (10) to (12), wherein thecircuitry comprises a first digital buffer (DBA) for driving a firsttrace of the unit pixel (UP1, . . . , UPN) and a second digital buffer(DBB) for driving a second trace of the unit pixel (UP1, . . . , UPN),and wherein to the first buffer (DBA) a first trace modulation signal(GDA) is supplied and wherein to the second buffer (DBB) a second tracemodulation signal (GDB) is supplied.

(14) The electronic device of (13), wherein the first digital buffer(DBA) and the second digital buffer (DBB) are enabled/disabled accordingto the multi-level mixing scheme to generate the effective first tracemodulation signal (Effective_GDA) and the effective second tracemodulation signal (Effective_GDB).

(15) A time-of-flight system comprising the circuitry of claim 1, alight source (2) and an image sensor (6).

(16) A method, comprising driving a unit pixel (UP1, . . . , UPN) for atime of flight camera according to a multi-level mixing clock scheme.

(17) A computer program, comprising instructions, the instructions whenexecuted on a processor controlling a driver of a unit pixel (UP1, . . ., UPN) for a time of flight camera according to a multi-level mixingclock scheme.

(18) A non-transitory computer-readable recording medium that storestherein a computer program product, which, when executed by a processor,causes the method according to (15) to be performed.

1. An electronic device comprising circuitry configured to drive a unitpixel for a time of flight camera according to a multi-level mixingclock scheme.
 2. The electronic device of claim 1, wherein the unitpixel comprises a first trace and a second trace and wherein themulti-level mixing scheme comprises supplying an effective first tracemodulation signal to the first trace of the unit pixel and supplying aneffective second trace modulation signal to the second trace of the unitpixel.
 3. The electronic device of claim 1, wherein the multi-levelmixing clock scheme is an N-level mixing clock scheme, wherein theeffective modulation signals have N+1 voltage levels, where N is aninteger number greater than
 1. 4. The electronic device of claim 3,wherein in the N-level mixing clock scheme the N+1 voltage levels of theeffective modulation signals define N voltage steps.
 5. The electronicdevice of claim 2, wherein the multi-level mixing clock scheme is atwo-level mixing clock scheme, wherein the effective modulation signalsprovides three voltage levels.
 6. The electronic device of claim 1,wherein the multi-level mixing scheme is an active multi-level mixingscheme.
 7. The electronic device of claim 6, wherein the activemulti-level mixing scheme comprises generating an effective first tracemodulation signal and an effective second trace modulation signal frompredefined voltage levels.
 8. The electronic device of claim 6, whereinthe circuitry comprises switches which are driven according to themulti-level mixing scheme to generate the effective first tracemodulation signal and the effective second trace modulation signal. 9.The electronic device of claim 6, wherein the voltage levels areprovided by analog buffers to the unit pixel.
 10. The electronic deviceof claim 1, wherein the multi-level mixing scheme is a passivemulti-level mixing scheme.
 11. The electronic device of claim 10,wherein the passive multi-level mixing scheme comprises passivelyredistributing charge between a first trace and a second trace of theunit pixel to generate the effective first trace modulation signal andthe effective second trace modulation signal.
 12. The electronic deviceof claim 10, wherein the circuitry comprises a switch that is configuredto connect a first trace of the unit pixel with a second trace of theunit pixel for passively redistributing charge between the first traceand the second trace of the unit pixel.
 13. The electronic device ofclaim 10, wherein the circuitry comprises a first digital buffer fordriving a first trace of the unit pixel and a second digital buffer fordriving a second trace of the unit pixel, and wherein to the firstbuffer a first trace modulation signal is supplied and wherein to thesecond buffer a second trace modulation signal is supplied.
 14. Theelectronic device of claim 13, wherein the first digital buffer and thesecond digital buffer are enabled/disabled according to the multi-levelmixing scheme to generate the effective first trace modulation signaland the effective second trace modulation signal.
 15. A time-of-flightsystem comprising the circuitry of claim 1, a light source and an imagesensor.
 16. A method, comprising driving a unit pixel for a time offlight camera according to a multi-level mixing clock scheme.
 17. Acomputer program, comprising instructions, the instructions whenexecuted on a processor controlling a driver of a unit pixel for a timeof flight camera according to a multi-level mixing clock scheme.